1. Field of the Invention
The present invention relates to a process for producing oil-in-water emulsions of hydrophobic materials.
2. Description of the Prior Art
Oil-in-water emulsions have been used in photosensitive materials, cosmetics, in making chemicals and pressure-sensitive recording materials, etc., in which the hydrophobic material is an active component. In the case of photographic materials, the hydrophobic materials are dye image forming compounds (herein couplers are used as a representative example), diffusion transfer or dye image forming compounds, anti color-fogging agents, antifading agents, antistaining agents, ultraviolet ray absorbing agents, whitening agents, etc.
In conventional processes for preparing oil-in-water emulsions for photosensitive materials and particularly silver halide photosensitive materials, a relatively small amount of an oily phase solution is added to a comparatively large amount of aqueous phase solution (containing binder) with stirring. If the hydrophobic material is liquid, the hydrophobic material is used as it is or as an oily phase solution wherein the hydrophobic material is mixed with an organic solvent, an emulsifying agent or a mixture of an organic solvent and an emulsifying agent. When the hydrophobic material is solid, the hydrophobic material is used as an oily phase solution wherein the hydrophobic material is dissolved in an organic solvent alone or a mixture of an organic solvent and an emulsifying agent. The resulting oily phase solution of the hydrophobic material is then added to and dispersed in an aqueous phase solution of a water-soluble binder with stirring. The aqueous phase solution of a water-soluble binder contains an emulsifying agent, if necessary. Thus, an oil-in-water emulsion having an average particle size of about 0.1 to 10 .mu.m is conventionally obtained.
Conventional processes for preparing oil-in-water emulsions for photographic sensitive materials have been carried out by means of an apparatus such as shown in FIGS. 1 (A) and 1 (B). In FIGS. 1 (A) and 1 (B), the coupler or the coupler and an emulsifying agent are first dissolved by mixing with an organic solvent in a first tank 1 equipped with a propeller stirrer 3 having a comparatively simple structure to produce an oily phase coupler solution. On the other hand, an aqueous solution of the hydrophilic colloid such as gelatin or gelatin and an emulsifying agent is prepared in a second tank 2 equipped with a propeller stirrer 4. Thereafter, the resulting coupler solution in the first tank 1 is added to the aqueous solution of gelatin in the second tank 2 [FIG. 1 (A)] or the coupler solution and the aqueous solution of gelatin are simultaneously poured into a third tank 6 equipped with a propeller stirrer 7 [FIG. 1 (B)] to produce an oil-in-water coupler emulsion. The resulting emulsion is then processed by an emulsifying machine 5 such as a colloidal mill, a homogenizer, a homomixer, etc., to make the particle size of the drops of the coupler solution a uniform suitable size. Thus, the emulsification process is accomplished.
By carrying out the aforementioned process, it is possible to disperse the couplers in the aqueous solution of gelatin (the continuous aqueous phase) to provide a particle size distribution such as shown by the dotted line for Curve I in FIG. 2. The average particle size of Curve I can be adjusted by selecting the type of the emulsifying machine or adjusting the number of times the emulsion is passed through the emulsifying machine.
The above-described emulsification procedure has the following drawbacks:
(1) Depending on the type of hydrophobic material which is dissolved in the drops of the oily phase solution it easily precipitates.
(2) It is difficult to prevent formation of large particles and to reduce the breadth of the particle size distribution range as shown in Curve I.
(3) The particle size sometimes grows when the oil-in-water emulsion is allowed to stand for a long period of time.
(4) It is very difficult to change average particle size as desired without changing the emulsifying machine that is used or the operating conditions.
(5) Since the coupler solution and the aqueous gelatin solution are each separately prepared in a first tank 1 and a second tank 2 and they are then mixed in a second tank 2 or a third tank 6, the coupler solution adheres to the walls of the first tank 1, the shaft or the propeller of the propeller stirrer 3, the inner wall of conduit 8 provided between the first tank 1 and the second tank 2, or the third tank 6 and, operation efficiency is not suitable because a long period of time is required for cleaning.
(6) Cleaning and adjustment of equipment such as the second tank 2 and the emulsifying machine 5, etc., sometimes takes much time and it is difficult to make up for the time loss, because the process comprises many steps.